Add separate AggregateSource.Testing.NUnit package

To make a super-duper-happy-path from NuGet, I've separated out the NUnitExtensions for AggregateSource.Testing from the sample and moved them into their own library, since it seems that this is something every project is going to need.

Will also do a version targetting xUnit.net for those who are that way inclined (I assume no-one uses MSTest, though may be wrong there!)

Summary of tasks completed below:

- [x] separate NUnitExtensions into own library
- [x] add XML documentation to public members of NUnitExtensions
- [x] update SampleSource to use new library
- [x] add new library to build
- [x] add .nuspec for AggregateSource.Testing.NUnit
- [x] add AS.Testing.NUnit to Nuget Pack and Push target

All tested up until the "push" phase, and the packages with dependencies work correctly from a local NuGet server.